1. Field the Invention
The present invention relates to a camera apparatus having an optical low-pass filter. More particularly, the present invention relates to a camera apparatus having an optical low-pass filter suited for processing images one-dimensionally and discretely, as in video cameras, electronic still cameras or the like, using color image pick-up elements.
2. Description of the Related Art
In a photographing lens using image pick-up elements, such as CCDs, which obtain output images by discretely sampling image information, if spatial frequency components exceed the frequency limit of the image pick-up elements then, when an object is photographed, a phenomenon, such as a so-called beat, occurs in the prior art. In this situation, moires which have originally not been present in the object appear, or compacted moires appear as thick moires having uneven densities. That is, frequency components which cannot be picked up by a camera apparatus cannot be reproduced as image information, and therefore a phenomenon called a so-called waveform distortion (areaging) occurs.
To suppress this areaging, in a photographing lens of, for example, a video camera, an optical low-pass filter is disposed in the light path of the photographing lens so that the light flux from the object is separated into a plurality of directions. This is accomplished by permitting the light flux to pass through the optical low-pass filter, resulting in a single point image formed on an image photographing plane that is separated into a plurality of point images. As a result, the influence of the areaging is suppressed by limiting the high-band frequency characteristics of the object.
As for optical low-pass filters, various filters using birefringence produced by uniaxial crystals, such as crystallized quartz, or filters in which a diffraction grating is disposed in the light path of a photographing lens and the diffraction effect of the diffraction grating is utilized, are used. Since optical low-pass filters, in particular those using a diffraction grating, can be produced easily by molding plastics, they can be obtained at quite a low price. Therefore, they have been widely used in the photographing lenses of recent video cameras or the like. Optical low-pass filters using a diffraction grating have been disclosed in, for example, Japanese Patent Laid-Open Nos. 53-119063 and 63-287921.
In addition, there has been a demand for a camera apparatus, for use with a commercial-use video camera, which has an optical low-pass effect and high optical performance, and in which the entire photographing lens is made compact since the entire video camera has become compact.
Generally, to maintain a satisfactory optical performance of a photographing lens and at the same time reduce the number of lenses and make the entire lens system compact, it is effective to use aspherical lenses. For example, Japanese Patent Laid-Open No. 2-39011 discloses a compact photographing lens using aspherical lenses. However, aspherical lenses have a problem in that their optical characteristics change greatly due to changes in the environment, such as temperature or humidity, though they can be produced relatively easily by using plastic molds.
By contrast, if aspherical lenses are produced by using glass molds, variations in their optical characteristics due to changes in the environment, such as temperature or humidity, decrease. However, there is a problem in that it becomes difficult to produce them, since, for example, the types of grinding materials which can be used are limited, and maintaining the surface precision satisfactorily is difficult.